The heart is the center of a person's circulatory system. It includes an electro-mechanical system performing two major pumping functions. The left portions of the heart draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart draw deoxygenated blood from the body organs and pump it to the lungs where the blood gets oxygenated. These pumping functions are resulted from contractions of the myocardium. In a normal heart, the sinoatrial node, the heart's natural pacemaker, generates electrical impulses at a normal cardiac rhythm. The electrical impulses propagate through an electrical conduction system to various regions of the heart to excite the myocardial tissues of these regions. Coordinated delays in the propagations of the electrical impulses in a normal electrical conduction system cause the various portions of the heart to contract in synchrony to result in efficient pumping functions. A blocked or otherwise abnormal electrical conduction and/or deteriorated myocardial tissue cause dysynchronous contraction of the heart, resulting in poor hemodynamic performance, including a diminished blood supply to the heart and the rest of the body.
Implantable pacemakers are used to restore cardiac rhythm and/or synchrony of cardiac contractions by delivering electrical stimulation pulses, known as pacing pulses, to one or more pacing sites in the heart. The minimum level of pacing energy required for a pacing pulse to excite the myocardial tissue in a pacing site is known as the pacing threshold for that pacing site. During implantation of an implantable pacemaker, the pacing threshold is evaluated with respect to each pacing site. The pacing energy delivered by the implantable pacemaker with each pacing pulse to each pacing site is then determined as a value that exceeds the pacing threshold by a safety margin. Under certain circumstances, the pacing energy so determined exceeds the maximum pacing energy level deliverable with each pacing pulse by the implantable pacemaker. This may happen, for example, when the tissue property is altered by a pathological condition or event, such as myocardial infarction. Myocardial tissue in an infarct region generally becomes less electrically excitable, requiring an increased energy level to activate.
Additionally, because the implantable pacemaker is battery-powered, delivering pacing pulses at lower energy levels increases the device longevity. For these and other reasons, there is a need to manage the pacing threshold at each pacing site to improve efficacy and efficiency of a pacing therapy.